Diesel engine exhaust has been classified as cancer-causing by the World Health Organisation's cancer research agency on Tuesday and the UN health body has urged action to reduce human exposure to it.
"Diesel engine exhaust causes lung cancer in humans," said Christopher Portier, chairman of a working group at the WHO's International Agency for Research on Cancer (IARC).
There was also a "positive association" with an increased risk of bladder cancer, said the UN body.
"Large populations are exposed to diesel exhaust in everyday life, whether through their occupation or through the ambient air," said a statement.
People are exposed to the exhaust gases of diesel cars, trains, ships and power generators.
"Given the additional health impacts from diesel particulates, exposure to this mixture of chemicals should be reduced worldwide," said Portier.
In 1988, the IARC had classified diesel exhaust as "probably" carcinogenic.
Portier's group also concluded on Tuesday that petrol, or gasoline, exhaust was possibly carcinogenic, a finding unchanged from its previous assessment in 1989.